The present invention relates to a key telephone transmission system in which a power supply line, two speech paths and a control signal path are formed by four wires.
In recent years, attempts have been made to reduce the number of cable conductors of a key telephone system but, with the progress of the key telephone system, transmission and reception of more signals have become necessary between key telephone sets and the key service unit. The signals to be transmitted and received are a speech signal, a control signal and a power supply and, for transmitting them with a four-conductor line, various arrangements are employed. For example, two conductors are used for transmitting the speech signal and the power source and the other two conductors are used for the control signal, or two conductors are used only for the speech signal and the other two conductors are for the power supply and the control signal.
In general, the key telephone transmission system is selected from the viewpoints of whether or not it possesses required transmission performance and whether or not it permits simplification of hardware for the terminal equipments of subscriber's lines and key telephone sets.
On the other hand, there has been proposed, as a result of improved function of the key telephone system, such an arrangement that has one more speech path, that is, employs six conductors, and uses one of the speech path for an ordinary calls and the other as an auxiliary speech path. With the provision of this second speech path, it is possible to very smoothly perform such a function as a camp-on function through using the calling party's voice instead of a mere tone signal. However, an increase in the number of transmitted and received signals is inconsistent with the reduction of the number of cable conductors used.